A pump/probe experiment is a commonly used method for performing time-resolved measurements on ultra-fast time scales. Two pulses (i.e., a pump pulse and a probe pulse) are typically used to investigate a system. The pump pulse is used to excite a response in the system. The probe pulse, which is delayed with respect to the pump pulse, is used to gather information about the excited system at a particular time delay. By varying the time delay between the pump pulse and the probe pulse, information about the temporal response of the system to the pump pulse may be obtained.
A time delay between the pump pulse and the probe pulse may be achieved by reflecting one of the pulse trains off a slowly moving retro-reflector mounted on a motorized translation stage. The signal is obtained by chopping one beam and using a lock-in amplifier (LIA) referenced to a chopping frequency to minimize source noise in the passband of the LIA. Using a slowly moving retro-reflector is usually referred to as a slow scan technique.
Another technique is rapid, repetitive scanning and signal averaging. Many fast scans can be taken and averaged together. Signal fluctuations are averaged or filtered to reduce noise. Depending on the number of signals that are averaged, several orders of magnitude reduction in noise may be achieved.
Typical rapid, repetitive scanning apparatus may have a scan rate of several tens of Hertz and may have scan ranges of approximately 150 picoseconds or less. Rapid, repetitive scanning apparatus may be mounted on a motorized translation stage to extend the scan ranges. Signals may be concatenated to create a single signal with a scan range longer than the scan range provided solely by the rapid, repetitive scanning apparatus. A typical design or control tradeoff in the rapid, repetitive scanning apparatus is the scan range versus the scan rate. Typically, as the scan range decreases, the scan rate may increase, and vice versa. Also, linearity of the delay may be a design or control tradeoff of the rapid, repetitive scanning apparatus.
FIG. 1 shows a diagram of a typical rapid, repetitive scanning apparatus (100). A retro-reflecting mirror (104) moves in movement (133) to a secondary position (108). The movement (133) is repeated so that the retro-reflecting mirror (104) translates between positions.
An optical input (101), or optical input beam, impinges on the retro-reflecting mirror (104). An incidence angle, θ, between the optical input (101) and a line normal to the retro-reflecting mirror (104) may be measured when the optical input (101), or optical input beam, impinges on the retro-reflecting mirror (104). An optical output (103) maintains the same incidence angle, θ, on an opposite side of the line normal to the retro-reflecting mirror (104). The optical output (103) impinges the retro-reflecting mirror (104) at a different location than the optical input (101), which results in an optical output (105).
The optical input (101) and the optical output (105) are parallel to each other. As the retro-reflecting mirror (104) sweeps forward and backward, the optical input (101) and the optical output (105) continue to be parallel to each other. For example, as the retro-reflecting mirror (104) moves to the secondary position (108), the optical input (101) continues along optical path (107). The optical input (101) impinges on the retro-reflecting mirror (104) at the secondary position (108) and is reflected along optical path (109). Furthermore, the optical input (101) is reflected from optical path (109) to optical path (111) to form the optical output (105). Accordingly, as the retro-reflecting mirror (104) sweeps forward and backward, an optical delay is changed depending on the distance an optical beam travels.
Ideally, an optical delay has a linear temporal delay. In other words, as the retro-reflecting mirror (104) sweeps forward and backward, the retro-reflecting mirror (104) moves at a constant velocity. If the movement (133) has a constant velocity, the temporal delay of the optical beam is linear. In the typical rapid, repetitive scanning apparatus (100), the retro-reflecting mirror (104) sweeps forward, stops, sweeps backward, stops, and repeats the forward movement. The retro-reflecting mirror (104) has a mass that must be stopped, then accelerated. Accordingly, the retro-reflecting mirror (104) may only move with a constant velocity during a portion of the forward and backward movement (133).
Furthermore, time is required for the retro-reflecting mirror (104) to sweep forward and backward. As the scan range (i.e., a distance traveled by the retro-reflecting mirror (104)) increases, the scan rate (i.e., the time required to travel the distance) decreases, and vice versa.